Some rotary actuators have an integrated capacitive sensor system. For example, DE 10 2004 010 406 B3 describes an information and entertainment system having a rotary actuator with an integrated touch sensitive surface. The touch sensitive surface implements a touch screen as an input device for handwriting recognition.
In order to design a touch sensitive surface on a rotary actuator, it is advantageous to arrange the electrical components of the sensor surface on the stator such that the electrical connectors of the components do not restrict the rotational capability of the rotor. The actuating surface itself can be actualized by the outer top surface of the rotor. Tactile contact of the actuating surface of the rotor affects the components of the sensor surface on the stator.
A spacing or interval is necessary between the inner surface of the rotor and the sensor surface on the outer surface of the stator. The spacing minimizes detrimental frictional effects for the stationary sensor surface under the rotatable actuating surface of the rotor.
The properties of touch sensitive capacitive sensors are dependent on the material arrangement between the actuating surface of the rotor, on which a human finger or the like can act, and the sensor surface on the stator. The specific relative permittivity εr and thickness of the material is of importance for a precise evaluation of the signal. A large difference exists between the relative permittivity of air (εr=1.0) and thermoplastic materials (εr=2.5-3.5) from which the components of the rotational actuator, such as the rotor, are typically made. Due to this property, the thickness of an air gap between the rotor and the sensor surface on the stator disproportionately affects the measured capacitance values.
A difficulty arises in that the relative position and spacing between the actuating surface of the rotor and the sensor surface on the stator cannot be maintained precisely during rotation of the rotor about the stator. Depending on the precision of the pivot bearing of the rotor, different portions of the actuating surface can have different distances to the sensor surface, which also change during rotation of the rotor. The electrical properties of the capacitive sensor are thus dependent on the actuating position and are not constant when the rotary actuator is rotationally activated.